Depend: Import 'pefile' only on windows.

This fixes building the documentation at read-the-docs:
Since 835fd2e 'pefile' was imported
unconditionally, even when running `pyinstaller --help`. And when
building the docs this very command is run to generate the input for
the man-pages. But 'pefile' is not available at read-the-docs and the
build fails.

Author: htgoebel

PyInstaller/depend/bindepend.py

@@ -15,7 +15,6 @@
 import os
 import re
 import sys
-import pefile
 from glob import glob
 # Required for extracting eggs.
 import zipfile
@@ -26,23 +25,22 @@
                       is_darwin, is_freebsd, is_venv, base_prefix, PYDYLIB_NAMES)
 from . import dylib, utils
 
-
 from .. import log as logging
 from ..utils.win32 import winutils
 
 logger = logging.getLogger(__name__)
 
 seen = set()
 
-# Do not load all the directories information from the PE file
-pefile.fast_load = True
-
 # Import windows specific stuff.
 if is_win:
     from ..utils.win32.winmanifest import RT_MANIFEST
     from ..utils.win32.winmanifest import GetManifestResources
     from ..utils.win32.winmanifest import Manifest
     from ..utils.win32 import winresource
+    import pefile
+    # Do not load all the directories information from the PE file
+    pefile.fast_load = True
 
 
 def getfullnameof(mod, xtrapath=None):